Amplitude range control in signal transmission systems



- March 14-, .1944. s wR|GHT 2,344,012

AMPLITUDE RANGE CONTROL IN SIGNAL TZ-RAN-SMISSION SYSTEMS Filed May 2, 1940 TC D DELAY -GAS FILLED SIDE BAND DELAy 124610 Y RECEIVER CARP/El? (COM-PARISON c/Rcu/T m/vm TOR 5. B. WRIGHT 'zlzfw ATTORNEY Patented Mar. 14, 1944 UNITED STATES PATENT OFFICE.

. AMPLITUDE RANGE CONTROL IN SIGNAL TRANSMISSION SYSTEMS Sumner B. Wright, South Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 2, 1940, Serial No. 332,893 6 Claims. (01.25040) The invention relates to the control of transmission in signal transmission systems and particularly to the control of th amplitude range of signals in such a system. In a non-linear transducer, the output power is not proportional to the input power. Consequently, the ratio of maximum to minimum power at the output differs from that at the input. The ratio of maximum to minimum power is an expression of amplitude range. 'A device designed to alter this range may be called a range controller. In telephony, the term'range controller includes many devices which have been designated with more specific names, such as volume limiters, volume control devices, range reducers, compressors, expanders, vogads (volumeoper-ated gain-adjusting devices) etc.

An object of the invention is to improve the operation of such devices.

- A more specific object is to improve the operation of a range controller in a telephone system subject to variable interference, such as line noise, static, or selective fading.

These objects are attained in accordance with the invention by circuits for making the time action of a range controller depend on the rate of variation of its input. In one embodiment applicable, for example, to a volume range exe pander or compressor, the time constant of the device is adjusted to provide for slow operation when the input has the nature of a strong, sude denly applied sound, such as a click of noise (short period), and fast operation when themput consists of waves of gradually changing amplitudes (long period), such as the syllabic variations in speech, so as to produce the desired operation on applied speech but not on applied noise.

Another embodiment is in an automatic gain control circuit for a radio receiver of the single side-band type, in which the time constant of the gain control circuit is made a function of the rate of change of the carrier envelope, so that the receiver output is improved when conditions change 'as a result of selective fadin The various objects and features of the inven tion will be better understood from the following detailed description when read in conjunction with the accompanying drawing in which:

Figs. 1 and 2 show schematically the invention embodied in a, range controller of general application, and, in an automatic gain control circuit for a radio receiver, respectively. 7 f

The amplitude range controller of Fig. 1 includes a vario-amplifier VA inserted in. a telephone circuit TC subject to variable interference such as line noise. The vario-amplifier VA includes an input transformer T1 having its primary winding connected to the incoming portion of the telephone circuit TC; an output transformer T2 having itssecondary winding connected to the outgoing portion of the telephone circuit TC; two three-electrode amplifying tubes V1 and V2 having their control grid-cathode circuits connected in push-pull relation through respective halves of the secondary winding of input transformer T1, and their anode-cathode circuits connected in push-pull-relation through the respective halves of the primary winding of output transformer T2; and a condenser C3 connected in series with the common portion of the control grid-cathode circuit of tubes Vrand V2, the variable voltage across which controls the gain of the vario-amplifier VA.

The circuit arrangement modified in accordance with the invention for controlling the volt age across the gain control condenser C3 includes the delay circuits D1 and D2 connected in tandem in the incoming portion of the telephone circuit TC in front of the vario-amplifier VA, th delay circuit D2 being the usual one provided for delaying the application of the signal waves to the input of the amplifier until after the forwardacting control circuit has completed the adjustment of its gain, whereas the delay circuit D1 has a different function in accordance with the invention, which will be described below.

The charge on gain control condenser C3 is controlled partly by a branch control circuit, including input transformer T5, full-wave rectifier F3 and th gas-filled control tube CT, having its input connected across the circuit TC between the delay circuit D1 and the delay circuit D2; and partly through the anode-cathode imped ance R5 of a three-electrode vacuum tube Va, the value of which impedance is controlledby the volt-age applied between the control grid and cathode of the latter tube from the output of a Comparison circuit, shown within the dot-dash box so labeled. 7

The comparison circuit consists of two, fullwave rectifiers F1 and F2, the inputs otwhich are respectively connected across the circuit TC through transformers T3 and T4, respectively, on the input and output sides of the delay circuit D1. The outputs of these rectifiers are connected in opposition respectively across equivalent con densers C1 and C2 connected in a closed series-circuit with equivalent series resistances'Ra and B4. A mid-point between the two condensers C1 and C2 and a mid-point between the two resistances R3 and R4 are connected to the control grid and ance R oftube Vabythese connections is made dependent on the amount the charge on condenser Cz exceeds the charge on condenser C1 in the comparison circuit. The anode-cathode impedance R5 of the tube V3 is. connected in series with condenser C4 and parallel resistanceRs and the cathode-to-anode impedances of the gas filled control tube CT across the gaincontrol condenser C3, and the output of: the. full-wave recti-.

fier F3 is connected across condenser C4. and parallel resistance Re.

The circuit arrangement of Fig. 1 operates as follows:

Let itybeassume'd, first, that a. short-period click-of noiseisapplied to the input of the telephone circuit. TC. A part of the noise voltage in the. input of. delay circuit D1 will be passed through transformer T3130 the comparison circuit, and will be rectified by full-wave rectifier Frtoproduce a proportional charge on the condenser C1. Assuming that the double-pole, double-throw switch-Sis operated to the left, this charge appliedbetween the control grid and cathodeof the tube V3 will tend to decrease the anode-cathode impedance R5 of that tube, thus tending;;to decrease the time constant of the gain control circuit of the vario-amplifier VA. However, a short time later when the voltage due, to the noise click appears at the output of. delay circuit D1, other portions will pass throughthe input. transformer T4 in the comparison circuit, and input transformer T5 in the main. gain control branch, which have their primarywindings connectedzin parallel across thecirc'uit TC at a point between delay circuits Dr and DzQthus actuating both full-wave rectifiers Fz and Fa.

"The charge applied by rectifier F2 on the condenser C2 in the comparison circuit will be in opposition to that previously placed by the noise click on the condenser C1 of that circuit, the

V3, thus increasing the value of the anodecathode. impedance R5 of that tube and thereby increasing the time constant of the varioamplifier gain control circuit.

, The output voltage of full-wave rectifier F3 is applied across resistance Rs in parallel with condenser C4 to the control discharge gap of gasfilled control, tube CT. The main discharge gap of tube CT acts practically as an open circuit with no voltage applied by F3; When F3 delivers a sufficient voltage. across R6, the control gap of tube ,CT breaks down which causes the main gap of control tubeCT to become conductive, so that partof the applied voltage. from F3 is placed in series with C3 and R5 and will place. a charging voltage across C3. By this time, the noise click will have passed throughthe delay circuit D2. and, because the time constant of the main control branch of vario-amplifier VA has been set toa high value in themanner just described, the gain changewill be too slow to respond to the click voltage until after the noise click has been dissipated in input transformer T1.

charge on C1 by this time having been diminished I due to the noise click' having passed through Now, let it be assumed that a longer period wave, such as a speech wave, is impressed on the input of the circuit TC. In the same way as above described for the applied noise click, a portion of the. speech voltages in the input of delay circuit D1 will be first applied through transformer T3 to the comparison circuit and will be rectified in rectifier F1 to apply a charge on condenser C1. After passing through the delay circuit D1 in the circuit TC, other portions will be applied through transformer T5 to rectifier F3. in the main gain control branch, and

through transformer T4 to rectifier F2 in the comparison circuit. The output of rectifier F2 will produce av charge on condenser C2 in the comparison circuit.

Because of the longer period of a speech wave, during --the building up of the wave, the difference between the charges produced on the condensers Crand C2 of the comparison circuit will be smaller than in the case of the shortperiodnoise click, and the difference voltage, therefore; will" apply only a slight negative bias to'the control grid of the tube V: so that its, anode-cathode impedance R5 will be smaller than before. This will cause the time constant of the gain control circuit of the vario-amplifier VA tobe made smaller and, by the time the speech waves. in the circuit TC reach the output of delay circuit D2, the gain of vario-amplifier VA will have increased due to the charge produced'on condenser C3 from the gas-filled control tube CT. Thus, it will be seen that the speech waves are more effective in increasing the gain of the vario-amplifier VA than the suddenly applied clicks. At the end of each speech sound, the charge on condenser C2 of the comparison circuit will predominate and therefore the impedance R5 of tube V: will be large, but owing to the lag in operation due to the varied time constant, the distortion will be less; than at the beginning of the sound.

If the range controller of Fig. 1 is used as a volume range expander, the circuit constants would be selected so that the vario-amplifier VA would normally have a low gain, and speech whenit is applied to the. input of the circuit wouldbemore effective in increasing this gain than would short-period noise, as is apparent from the above description of operation.

Ifthe double-pole, double-throw switch S in the circuit of Fig. 1 is thrown to the right, and the batteries and other circuit constants are selected to provide the proper bias and other characteristics in the vario-amplifier VA, the device of Fig. 1 may be arranged so that the variation in the ratio of the time constant to theperiod of the envelope tends to be smaller as the envelope. frequency varies, thus giving a more uniform operation than a device in which the time constant is fixed and the envelope period varies. Such. an arrangement might have application in a compressor at the sending end of a telephone circuit. To obtain the compressor action with the circuit of Fig. 1 it will be necessary to reverse the leads connecting the terminals of C3 to the grids and cathodes of the vario-amplifier tubes V1 and V2 so that the control voltage from V3 will efiect an increase in gain of VA when control tube CT operates. from increasing input;

'{Ihe operation of the-circuit arrangement of Fig. 1 so modified would be as follows: Assuming that the envelope period of the speech wave applied to the input of the circuit TC is short, at the instant the wave reaches the input of the vario-amplifier VA, the charge produced on the condenser C; will exceed that on the condenser C1 of the comparison circuit, and the value of the anode-cathode resistance R of tube V3 will consequently be low. This will permit the gas-filled control tube CT to operate with a small time constant.

If, on'the other hand, the syllabic period of the applied speech wave is long, there would be a smaller difference produced in the charges on the condensers C1 and C1 in the comparison circuit so that the impedance R5 would be correspondingly greater. Thus, the time constant obtained in the control for the vario-amplifier VA at the instant of the arrival of any portion of the applied wave in the output of delay circuit Dz would bear a proper relation to the period of the wave, that is, a long time constant with a long period.

Fig. 2 shows a modification of the invention applied to a carrier-operated automatic volume control for a single side-band radio receiver in which the time constant is made a function of the rate of change of the carrier envelope so that the output is improved when conditions change as a result of selective fading. The elements of the arrangement of Fig. 2 corresponding to those of Fig. 1 are identified with the same identification characters.-

As' shown in Fig. 2, the radio receiver RR has a side-band output circuit I corresponding to the telephone circuit TC in the system of Fig. 1, and a carrier output circuit 2 which is employed as a forward-acting circuit for controlling the gain of a vario-amplifier VA, corresponding to the amplifier so identified in the system of Fig. 1, connected in the side-band output circuit l by the input and output transformers T1 and T2 and preceded by a delay circuit Dz corresponding to the delay circuit similarly identified in the circuit TC in the arrangement of Fig. 1. The first delay circuit D1 in the system of Fig. 2, however, corresponding to the delay circuit so identified in the system of Fig. 1, is connected in the carrier output circuit 2 instead of in the main signal circuit as in Fig. 1. A Comparison circuit shown within the dot-dash box so identified, identical with that of Fig. 1, has the input of its rectifier F1 connected by transformer T3 across circuit 2 in the input of D1, and the input of its rectifier F2 connected by transformer T4 across the output of D1 as in the system of Fig. 1.

Also in Fig. 2, as in the system of Fig. 1, gain control condenser C3 in the common portion of the control grid-cathode circuits of the tubes V1 and V1 of the vario-amplifier VA obtains its 7 charge partly from a branch control circuit including an input transformer T5, full-wave rec tifier F3, and a gas-filled control tube CT, but having its input connected across the carrier output circuit 2 in the output of delay circuit D1, instead of to a corresponding point in the main signal circuit as in Fig. 1. In the system of Fig. 2, like in Fig. 1, the gain control condenser C3 obtains part of its charge through the anodecathode impedance R5 of a three-electrode Vacuum tube V3, connected in series with condenser 04 and parallel resistance Re, connectedacross the output of full-wave rectifier F3, and the main cathode-anode discharge gap of gas-filled control tube CT across condenser C3; an'dthe value of the anode-cathode resistance R5 of tubeVa is controlled by the difierence in the voltages of condensers C1 and C2 of the comparison circuit, applied between the control grid and cathode of the tube V3. 1

In addition, the circuit of Fig. 2 includes an electromagnetic threshold (marginal) relay S1 having its winding connected across the carrier output circuit 2 at a point in front of delay circuit D1 through the series half-wave rectifier F1 and shunt condenser Can The relay S1 is operated in response to normal carrier, but releases in response to the falling 011' of the carrier output of the radio receiver RR during a selective fading period to control relay S2 and associated timing circuits, to disable the charging circuit for the gain control condenser C3 of vario-amplifier VA, and to hold its disabled for a definite time interval-as required to prevent gain adjustment of the vario-amplifier VA during the time when the carrier is being restored to its normal value, in a manner which will be described below.

When the carrier in the carrier output circuit 2 of radio receiver RR, is normal, it applies through full-wave rectifier F3 connected across the output of delay circuit D1, a negative charge on gain control condenser C: which will adjust the gain of the vario-amplifier VA so that it is correct for its input. As 'the carrier amplitude decreases, the charge produced on gain control condenser C3 is reduced, causing the gain of the vario-amplifier .VA to be proportionately increased.

Assuming that the carrier begins to fade, the carrier output of delay circuit D1 will tend to become stronger than the carrier input to delay circuit D1 thus building up an excess charge on the condenser C2 in the comparison circuit through transformer T4 and rectifier F2, the effect of which is to make the anode-cathode impedance R5 of tube V3 small. This will cause the time constant of the amplifier VA to be small so that the control circuit acting through rectifier F3 produces gain changes rapidly following the carrier envelope. However, as the carrier drops below the threshold value for which the relay S1 is designed to release, no further change takes place in the time constant of the control circuits for the vario-amplifier VA or in the amplifier gain because relay S1 releases, causing relay S2 to be operated by current from the battery B3 supplied to the operating winding of the latter relay. Now, as the amplitude of the carrier increases at the end of the fade, the threshold relay S1 will reoperate. However, the release of the relay S2 is delayed for a period determined by'the values of the resistanceelements R1 and Re and the condenser element C5 while the latter condenser is being charged up by current from the battery B1 through a holding winding on relay S2. Thus, the change which wouldotherwise occur due to the carrier being restored to its normal value is not allowed to effect the time constant of the control circuit for the vario-amplifier VA.

Assuming now that theamplitude of the carrier in carrier output circuit 2 remains steady or drops off very slowly, it will be desirable to have a longer time constant on the gain control for the yario-amplifier VA. This effect is obtained by the comparison circuit because for that condition the difference between the charge produced on condenser 02 and condenser C1 is quite small, resulting in'a large value for the anodecathode impedance R5 ofthe tube V3 and thus providing a very slow variation in the gain of the vario-a'mplifier. v V

Other arrangements of the apparatus and circuits shown in Figs. 1 and 2 which will provide the desired variation of the time constants of the control for the variable gain amplifier to provide improved operation in the presence of noise or selective fading, which are within the spirit and scope of the invention, willoccur to persons skilled in the art. For example, it is apparent that the gas-filled control-tube CT and the vacuum tube V3 may be replaced by other known types of variable impedances, and that a backward-acting gain control circuit may be used with the range controller instead of a forward-acting circuit such as illustrated and described.

What is claimed is:

1. In combination with a range controller in a signal transmission circuit, including a variable gain amplifier in said circuit and a control circuit for automatically adjusting the gain of said amplifier to make the signal amplitude range in its output a desired function of the signal amplitude range in its input, means to improve the operation of said range controller in the presence in said circuit of interfering noise energy of relatively short period'compared to that of the signal waves, or selective signal fading conditions, comprising means responsive to the waves applied to control circuit to adjust its time constant so as to produce a slow variation in the gain of said amplifier for the applied waves of short period and a fast variation in its gain for applied waves of relatively long period.

2. The combination of claim 2; in which said amplifier includes an electron discharge device having a control grid circuit including a condenser, said gain-adjusting control circuit comprises means for selecting from the transmission circuit and rectifying a component part of the signal waves, and means to utilize the rectified waves to charge said condenser in accordance with their amplitude and thus produce a proportional variation in the amplifier gain, and said means for adjusting the time constant of said gain-adjusting control circuit includes a variable impedance connected in a closed circuit with said condenser, a delay circuit in the path of the waves applied to said gain-adjusting control circuit, means to compare the amplitude level of the waves in the output of said delay circuit with that of the waves in the input thereof, and means for varying the value of said variable impedance in a direction and amount dependent on the amplitude difference of the compared waves.

3. The combination of claim 2, in which said amplifier includes an electron discharge device having a control grid circuit including a condenser, said control gain-adjusting circuit is connected to the signal transmission circuit and includes means for rectifying the applied waves, and a variable impedance connected across said condenser, the value of which is varied in ac cordance with the amplitude of the rectified waves to control the charge on said condenser, and said means foradjusting the time constant of said gain-adjusting control circuit comprises a second variable impedance connected in series with the first variable impedance acrosssaid condenser, a delay circuit connected in said transmission circuit in front of the point of connection of said control circuit thereto, means for continuously comparing the amplitude .levels of the waves in the input and output of said delay denser, said control gaineadjusting circuit is connected to the signal transmission circuit and includes means for rectifying the applied waves, and a variable impedance connected across said condenser, the value of which is varied in accordance with the amplitude of the'rectifled waves to control thechargeon said condenser, and said means for adjusting the time constant of said control circuit comprises a second variable impedance connected in series: with the first variable impedance across said condenser, a

delay circuit in said transmission circuit in front of the point of connection of said control circuitthereto, means for respectively rectifying the voltage waves in the input and output of said delay circuit, two condensers connected in opposition and respectively charged in accordance with the amplitude level of the respective rectified voltage waves, and means responsive to the differences in the charges on the two condensers to change the value of said second variable impedance proportionately, V

5. The combination-of claim 2-, in which said range controller is a carrier-operated automatic gain control for a radio-receiver, said amplifier being connected in the signal side-band output circuit of said receiver, and including an electron discharge device having a control grid circuit including a condenser, said control circuit being selective to the carriercomponent of the waves in the output of the radio receiver ,and comprises means for rectifying said carrier component, and a variable impedance connectedacross said condenser the value of which is varied in accordance with the amplitude of the rectified wave to control the charge on said condenser, as to vary the gain of said discharge device, and said means to adjust the time constantof said control circuit comprises a second variable impedance element connected in series with thefirst variable impedance element across said condenser so as to vary the charge on the latter in accordance with its value, a delay circuit in said control circuit in front ofthe rectifying means, means for continuously'comparing the amplitude levels of the waves in the input and output of said delay circuit, means responsive to difference inthe amplitude levels of the compared waves to'proportionately adjust the valueof 'said second variable impedance to make the time constant of said control circuit a function of the carrier envelope, and means responsive to the carrier output of said receiver to prevent the gain of said electron discharge device from being varied during a period of selective fading and the subsequent period during which the carrier is being restored to its normal amplitude value.

6. In combination with an automatic gain control for agradio receiver comprising a variable gain electron discharge device having a control grid circuit including a gain control condenser; connected in the, side-band output of said receiver and a control circuit responsive to the carrier output of said receiver to vary the charge on said condenser in accordance with the carrier amplitude variations,- meansto improve the opthe carrier envelope, and means to prevent the grain of said electron discharge device from being varied during a period of selective fading and the subsequent period during which the carrier 5 is being restored to its normal amplitude value.

SUMNER B. WRIGHT. 

